1. Field of the Invention
The present invention relates to a flow control device for controlling the flow rate of various kinds of fluids. Specifically, the present invention relates to a control device for a motor which drives a valve mechanism of the flow control device.
2. Description of Related Art
A flow control device is used to control the flow rate of gas or a liquid, for example, an energy medium such as liquefied petroleum (LP) gas, town gas or oil, or a coolant or warm water, which are used in equipment such as a water heater, a refrigerator or an air-conditioner. Regarding the mechanism for controlling the flow rate of gas or a liquid in the flow control device, a valve mechanism is used in which a valve element is driven by a solenoid or a motor. However, the solenoid driven system controls the valve element only to be opened or closed, and thus the flow rate cannot be controlled with a fine adjustment and a high degree of accuracy. Also, when the valve element is closed, an impact sound generates. On the other hand, a new system is proposed wherein a valve driving device with a motor as a driving source relatively moves a valve element with respect to an opening part. Thereby, the flow rate can be controlled with a high degree of accuracy in comparison with the solenoid driven system.
A stepping motor is used as the motor which is capable of controlling the rotational angle by applying a specified number of driving pulses. The stepping motor is provided with a detent torque which makes a rotor stop by using a magnetic attraction force with a stator. Therefore, after the specified number of driving pulses are applied to the motor in order to rotate and drive the rotor, the energization to the stepping motor stops and a stopped position is maintained by the detent torque. However, the stepping motor used in the valve driving device is designed so as to shut tightly between the rotor and the stator with a shielding member in order to prevent the leakage of a fluid such as gas, and thus the clearance between the rotor and the stator becomes large, which causes the detent torque to decrease. In this case, when the energization of the stepping motor stops, the damping time becomes longer and it takes a longer time to stop the rotor and further its stop position becomes unstable.
In other words, assume that, for example, after the specified number of driving pulses has been applied to the motor in order to obtain a prescribed flow rate, and then the driving pulse stops. When the drive pulse stops at the time when the magnitude of the damping of the rotor does not become smaller than that of the force of the detent torque, below which the position between the rotor and the stator can be maintained, the rotor may rotate due to the damping of the rotor to a different position without stopping at the specified position and the stop position may shift as shown in FIG. 8. In this case, the position of the valve element shifts according to the deviation of the stop position of the rotor, and thus the flow rate becomes different from the prescribed value. Also, the control, for example, temperature control, cannot be attained with a high degree of accuracy. Moreover, due to the deviation of the stop position of the rotor, the stepping motor may not start due to stepping-out even when driving pulses are applied to the motor.
A stepping motor driving device is proposed as a device for preventing stepping-out and for starting surely. This stepping motor driving device provides main pulses for driving the stepping motor and a stop position reset pulse, which are applied as driving pulses, and the frequency of the stop position reset pulse is set to be different from that of the main pulse. At the initial stage of a start operation from a stop state, by energization to the motor, one pulse of the stop position reset pulse is applied for making the motor return to the prescribed stop position. Thereby, stepping-out is prevented when the stepping motor starts.
As described above, when the stepping motor is used in the valve driving device, the stop position of the rotor may be unstable when the energization stops. Therefore, stepping-out may easily occur when the motor starts from the stop state with no energization and the flow rate may become different from a prescribed value due to the positional displacement of the valve element. The stepping motor drive device described above prevents the problem of stepping-out as described above. However, the above-mentioned drive device fails to control the rotor to stop at the specified position. Accordingly, the position of the valve element of the valve drive device may shift due to the displacement of the stop position of the rotor, and thus the flow rate may become different from the prescribed value.